We propose to localize differential, optical absorption within biologic, tissue-mimicking phantoms using a broadband, pulsed, light source and a focused, ultrasonic transducer employed in a "receive only" mode. The imaging strategy that we will explore exploits the "photo-acoustic" effect, which describes the process by which ultrasonic pressure waves are created whenever short-duration, localized optical absorption takes place within an optical-absorbing medium. The proposed approach is an alternative to purely optical methods for localizing optical absorption within turbid media - Optical Diffusion Tomography (ODT). The potential advantage of photo-acoustic ultrasound over ODT is that sites of optical absorption are not obscured by optical scattering within the absorbing medium. Such sites are localized on the basis of the better-behaved, and well-understood, ultrasonic propagation properties of biologic media. While the proposed imaging approach, if successfully demonstrated, is potentially applicable to any imaging task for which ultrasound is currently used, we will only explore its applicability to mammographic imaging in this pilot project. We have chosen breast imaging because of the P.I.'s continued interest in breast cancer detection using non- ionizing radiation, and because the breast represents biologic anatomy that is well-suited to the proposed methodology. Through a series of experiments, we will establish the relationship between photo-acoustic signal size, incident optical energy, spatial resolution, % regional absorption and "lesion" size. From these measurements and our knowledge of the optical and ultrasonic properties of breast tissue, we will be in a position to predict the physical limitations of photo-acoustic ultrasonography if applied to the breast.